Part 2 in my series on American signals intelligence satellites of the 1960 will appear later today. It is about AFTRACK. Only one poor quality AFTRACK payload photo has been released. However, we have a bunch of photos of Agenas being processed and their AFTRACK payloads are probably visible, if only we knew what they looked like. I think that the gray box in the top photo may be TAKI and/or SOCTOP. TAKI was a signals intelligence payload designed to intercept Soviet Tall King air surveillance radars.

By fall 1959, a number of CORONA photo-reconnaissance spacecraft had already been launched under cover of the Discoverer program, but none had operated successfully. Program officials became concerned that the Agena spacecraft that carried CORONA might be vulnerable to tracking by Soviet radars, or possibly even deliberate electronic interference. They did not think this explained CORONA’s early string of failures, but it was a possibility they worried about. At the time, Harold Willis was working in the Office of ELINT located at CIA Headquarters when CORONA officials briefed him about their program and told him about their concerns.

Willis also learned about the Samos Subsystem F signals intelligence satellite program, which at the time consisted of the F-1 and F-2 payloads. The former was a relatively small payload that would fly attached to a Samos photo-reconnaissance satellite and the latter a larger and more capable payload that would occupy the front end of an Agena spacecraft. Although the specialized F-2 satellite might be able to detect Soviet transmissions or interference, it was then scheduled to fly years after CORONA became operational. Willis thought that the Soviet threat to CORONA and other military satellites could develop sooner and they should not wait for the Samos signals intelligence satellites to provide data. He was not simply worried about problems over the Soviet Union but even far out over the oceans: the Soviets also had ships and trawlers with radomes, and nobody knew what they were for.

Willis discussed his concerns with Lockheed’s Bill Harris, who was working on Subsystem F payloads. Willis concluded that the Agena upper stages carrying CORONA cameras should be equipped with a small payload for detecting Soviet radar tracking or interference with the spacecraft’s S-band beacon. The beacon was used to announce the satellite’s presence to American ground tracking sites so that they could communicate with it. CIA officials approved Willis’ proposal in November 1959. The Agena spacecraft had an aft rack to which instrument boxes could be attached, like a luggage rack on a car. “AFTRACK” became the collective name for a large number of payloads carried on numerous spacecraft during the first half of the 1960s.

By early 1968, the United States military was involved in an escalating ground and air war in Vietnam. American aircraft were being shot down at the rate of nearly one a day, and Operation Rolling Thunder, the bombing of North Vietnam, was in full swing as B-52s unloaded racks of bombs over the jungle. The US Air Force was engaged in a constant battle against Vietnamese SA-2 surface to air missiles (SAMs), jamming them and spoofing them, electrons dueling invisibly in the air. American airmen with the job of physically destroying the missiles, going by the name Wild Weasels, went into battle with patches on their shoulders bearing the acronym “YGBSM.” It was reportedly the response of one electronic warfare officer when first told what he would be doing: “You gotta be shittin’ me…” he said. Fighting SAMs was a brutal business, and eventually the US military brought its top secret signals intelligence (SIGINT) satellites into the war, using a strategic asset for tactical purposes. That effort followed over a half-decade advance in electronics and spacecraft technology.

Great article. Do you plan to make a part on the geostationary signal intelligence satellites?

I was not really planning on it. Part 4 is on the P-11 and other subsatellites. Will also touch on CANYON and I may also include JUMPSEAT (I have some info on both). Not sure if I'll go into RHYOLITE. There is minor new info on that, but not a lot. I will also do a separate article on the BIT boxes designed to protect the satellites from interference.

This is the image that accompanied my recent article. I got this declassified and it was exclusive to TSR. Never before seen image of the STRAWMAN satellite.

Whereas many of the payloads are labeled, there are actually two antennas that are NOT labeled. Based upon their positions, it appears as if one of those antennas may have been labeled and the label was deleted for security reasons. I thought at first that this might be CONVOY, but a list of payloads for the STRAWMAN missions indicates that only the first two carried CONVOY and this is clearly a later satellite.

CONVOY was a payload designed to intercept signals associated with an anti-ballistic missile (ABM) radar. HARVESTER was also designed to intercept signals associated with the SA-5 radar at Tallinn, Estonia. The SA-5 was initially suspected to be an ABM system. So it seems possible that HARVESTER was the follow-on to CONVOY.

Ergo, whatever those two antennas are, they are probably not follow-ons to CONVOY. They're something different. According to Gunter's Space Page "Strawman 4 also carried the RM 19 radiometer payload." I don't know where that information comes from. I also don't know what a radiometer would look like. But maybe that's one of those things here.

I'm intrigued by the "Reaper" antenna array. Were they spiral log periodic or spiral conical antennas? Spiral log periodics, which have a spiraling antenna conductor on a cone, are broadband and thus might make sense for this application. Helicals are probably designed for a tighter frequency band. The drawing suggests cones to me.

Now I'm trying to imagine how a GEO version of this might look. Big reflector onto an array of antennas, possibly.

We have a lot more on them now. "The SIGINT Satellite Story" was my primary source for my series (except for the AFTRACK stuff), but I've pieced together other information as well. The official history has some problems (for instance, referring to mission numbers and also Agena numbers, which is confusing), so you have to patch it together. My suspicion is that now that NRO declassified that chapter of the official history they may release a document collection on the Agena SIGINTS as well, just as they did for Dual Mode GAMBIT, QUILL, AFTRACK and others. Let's keep our fingers crossed, because there are a number of interesting questions that the history raised that I would like to be able to answer. Off the top of my head, here are some of the open questions:

-what were the various payloads for? There are a few that we just don't have any description about.-what were the frequency ranges in general?-how did the Agena satellites fit with the other satellites in operation, like POPPY? What was the division of labor and the overlap in their missions?-how did the Agena work for these satellites? We have a lot of info on the CORONA Agena development, but how did it differ for these satellites?-what were the configurations for each of the satellites? It appears that each satellite was different, but how?-what did the satellite payloads look like?-how did the PENDULUM system work? What was its value?-what happened to STRAWMAN 5? Was it preserved or scrapped?-why was the program eliminated? What had changed?

I've got partial answers to a lot of those questions, but nothing definitive. I'd like to nail that down more.

So REAPER was apparently primarily a location locator, and at least one sentence in the official history describes it as having sectors. So apparently it looked down and had (maybe) four quadrants. Depending upon where a signal originated, it could localize that signal, figuring out where it was on the Earth.

How it did this stuff is beyond me. There isn't much description in the official history, and I bet that it gets into weird frequency magic, so I'd probably never understand it anyway. Presumably the satellite itself has to know its location fairly precisely in order to pinpoint something on the Earth. But how did the satellite do that? And that goes to the earlier question I posed about the Agena: how did the Agena record north/south/east/west and up and down so that it knew the location of the signals it was picking up?

Ergo, whatever those two antennas are, they are probably not follow-ons to CONVOY. They're something different. According to Gunter's Space Page "Strawman 4 also carried the RM 19 radiometer payload." I don't know where that information comes from. I also don't know what a radiometer would look like. But maybe that's one of those things here.

"RM-19 - These infrared sensors were packaged and mounted as a secondary payload on an Air Force satellite for the purpose of collecting background data in three different spectral regions. This successful program was performed by Lockheed from start to launch in a period of ten months and required interfacing with the integrating contractor, the SCF and the AFWTR operations."

Here is also some info on the RM-19 payload (although focused on the cryocooler of RM-19 and not the radiometer itself) and the launch month: "July 1971" - with STRAWMAN-4 being the only Air Force launch in July 1971.

Based on your article, I am wondering if it was something they did on the ground to get the radar sites to switch on. You know, hand a pilot the task of zipping over the Vietnamese country side at high speed at a set time to get the radars to switch on while the satellite was overhead.

So REAPER was apparently primarily a location locator, and at least one sentence in the official history describes it as having sectors. So apparently it looked down and had (maybe) four quadrants. Depending upon where a signal originated, it could localize that signal, figuring out where it was on the Earth.

How it did this stuff is beyond me. There isn't much description in the official history, and I bet that it gets into weird frequency magic, so I'd probably never understand it anyway. Presumably the satellite itself has to know its location fairly precisely in order to pinpoint something on the Earth. But how did the satellite do that? And that goes to the earlier question I posed about the Agena: how did the Agena record north/south/east/west and up and down so that it knew the location of the signals it was picking up?

Whenever an array of antennas is used, it becomes possible through various means to detect the direction of an incoming signal. With some methods it is possible to do this without moving the antenna array itself. Transmitted signals can also be aimed in specific directions by arrays. Today's phased array radar is an example of the technique. It is all black magic to most of us who are not antenna experts.

Based on your article, I am wondering if it was something they did on the ground to get the radar sites to switch on. You know, hand a pilot the task of zipping over the Vietnamese country side at high speed at a set time to get the radars to switch on while the satellite was overhead.

PENDULUM was the code name for the activity of taking data collected from MULTIGROUP and later STRAWMAN satellites on the location of SA-2 emitters and putting it in the hands of people in Vietnam planning airstrikes. There is not much about it in the official history. However, I noticed that the time that it was implemented corresponded with the time when LBJ halted bombing North Vietnam, restricting airstrikes to below the 19th parallel (the border). My guess is that before LBJ did this, the USAF had a lot of assets over North Vietnam and that is how they detected the SA-2s. After the halt, they had much worse data on the SA-2 locations, so the satellites were used to provide some of that data. Probably not extremely useful, because the satellites were not overhead much. But every little bit helped.

Questions related to that include how did they do it? What was the equipment they used? How did they deal with the security requirement? How good was it in terms of accuracy?

The broader context is that it was rare to use "national" intelligence systems to support tactical battlefield requirements. So revealing PENDULUM is a surprise. I know of few other early uses of national level systems like this in the 1960s and 1970s.

What I don't have a good handle on is how the USAF hunted for the locations of SAM sites in North Vietnam. I know that in general Wild Weasel strike aircraft operated with other aircraft, like EB-66 Destroyers, to find the SAM sites:

I think that the EB-66s recorded radar signals and used direction finding techniques to figure out the location of the emitters. Then this data was fed back to command centers, along with data collected from lots of other sources, like other aircraft, U-2s, and photo reconnaissance. But I don't know how it was all coordinated. I assume that there was some component of the command centers that specifically had the task of identifying North Vietnamese air defenses and SAMs in particular. And the PENDULUM information would have gone to that component, probably in a classified manner with careful "need to know." So while everybody in the SAM location and hunting component might have had secret level clearances, there might have been a couple of guys who had clearances to know about the PENDULUM data--or at least cleared to know what it was. They might have then gone into the plotting room and said "We have data from PENDULUM that shows SAMs here, here and here..." Nobody else would know what PENDULUM was, but they might have guessed--accurately or not.

I'm speculating here. I have a bunch of books on the Wild Weasels and one or more of them might go into detail on how they identified targets and tracked them in the command centers so that they could send aircraft out to hunt them. I'll just have to look for that stuff.

So REAPER was apparently primarily a location locator, and at least one sentence in the official history describes it as having sectors. So apparently it looked down and had (maybe) four quadrants. Depending upon where a signal originated, it could localize that signal, figuring out where it was on the Earth.

How it did this stuff is beyond me. There isn't much description in the official history, and I bet that it gets into weird frequency magic, so I'd probably never understand it anyway. Presumably the satellite itself has to know its location fairly precisely in order to pinpoint something on the Earth. But how did the satellite do that? And that goes to the earlier question I posed about the Agena: how did the Agena record north/south/east/west and up and down so that it knew the location of the signals it was picking up?

Dwayne, it is clear that most of the systems on these satellites have one narrow reception beam pointing straight down. So when you hear the signal, you know that a radar is directly below the spacecraft. You get the location from orbital tracking. All the spacecraft needs is an accurate clock with time ticks recorded on the same tape as the radar signals.

REAPER seems to be different with multiple antennas. Since these are not arranged in any sensible phase relationship, probably they each point at a slightly different direction and produce a "rake" of beams across the ground track. This would give more precise locations.

This concept only works if the radar sends a significant amount of energy straight up. The standard Cosecant-squared antenna used in microwave radars has a hole in the beam at the zenith - in 1944 some USN vessels had a special up-looking radar installed to detect kamikazes diving through this hole.

But the Soviets in this period were mostly using VHF radars for early warning and these have a much broader beam in the vertical direction. In fact you can still see these huge antennas operating in the successor states today, often alongside modern narrow-band sets. Among their other virtues, they could pick up U-2s that were supposed to be flying above the microwave radars the CIA thought the Soviets were using.

In your SPACEFLIGHT article on this topic, there was a picture of a heavy ferret with rings of feed horns looking out sideways. This configuration would pick up the main horizontal beams from S-band and X-band radars.

In your SPACEFLIGHT article on this topic, there was a picture of a heavy ferret with rings of feed horns looking out sideways. This configuration would pick up the main horizontal beams from S-band and X-band radars.

I don't remember that image. I had one artist illustration of an early Agena SIGINT (labeled as Samos Subsystem F). I had no photos of the satellite payloads. I am trying to get photos of the payloads for future articles.